The tumor promoter, 12-0-tetradecanoylphorbol-13-acetate (TPA), and epidermal growth factor (EGF) inhibit the growth of human A431 epidermoid carcinoma cells within 24 to 48 hours after exposure of the cells to these agents. Addition of TPA and EGF inhibit cell growth in an aditive or synergistic manner. These effects on cell growth are preceeded by changes in type IV (basement membrane) collagen and laminin synthesis and secretion and by a change in the activity of a calcium-dependent, cyclic nucleotide-independent and phospholipid-dependent protein kinase (protein kinase C). Specifically, EGF produced a 2 to 3-fold stimulation in protein kinase C activity within 30 to 60 minutes following exposure to the cells. TPA alone had no effect on protein kinase C activity. However, TPA attenuated the increase in protein kinase C activity that was induced by EGF. In EGF-treated cells there was an enhanced phosphorylation of a 32, 39 and 81 KD soluble proteins while TPA enhanced the phosphorylation of 21 KD protein. Addition of TPA in the presence of EGF blocked the enhance phosphorylation of the 32 KD protein, but a new protein of 90b KD was found to be phosphorylated under these conditions. In vitro, it was observed that the 32 KD protein was specifically phosphorylated by protein kinase C. Within 2 hours after exposure of A431 cells to either EGF or TPA, there was a 2 to 3-fold increase in type IV collagen and laminin secretion and synthesis which was transient. Combination of EGF and TPA was more effective than either agent alone in promoting the secretion of type IV collagen and laminin but produced no further enhancement in the synthesis of either protein. Since the synthesis and deposition of and attachment to a basement membrane membrane by normal and transformed cells is a prerequisite for their subsequent proliferation and since a variety of growth factors can modulate the synthesis and/or turnover of various extracellular matrix components (i.e. type IV collagen and laminin), the regulation of the production of these components by EGF and TPA may relate to the ability of these agents to inhibit cell growth via their effects on protein kinase C.